1. Field of the Invention
The present invention relates to polyester film having excellent heat stability and mechanical strength which are prepared by using aliphatic polyesters having a melt viscosity of 2,000-100,000 poise at a temperature of 190.degree. C. and a shear rate of 100 sec.sup.-1, and having melting point of 70.degree.-200.degree. C. with biodegradability and sufficiently high molecular weights, such as number-average molecular weight of 20,000. Particularly, the present invention relates to T-die flat film, air-cooling inflation film and water-cooling inflation film, which are formed by the above aliphatic polyesters.
2. Discussion of the Background
Conventionally, it is no exaggeration to say that high molecular weight polyesters (referring to polyesters having number-average molecular weights of at least 10,000) generally used for films, fibers and the like are limited to polyethylene terephthalete, a condensate of terephthalic acid (including dimethyl terephthalate) and ethylene glycol.
However, even though these films, when they are in the form of a simple blown film, have high stiffness and high strength due to the molecular structure of the terephthalic acid, non-oriented films are too brittle to be used as a film, and thus the films have been widely used after being had been oriented. Although oriented polyester film shows excellent transparency and strength, it has inferior heat sealing properties and thus for providing a heat seal packing use film, oriented polyester film has only been used in the form of a laminate with a polyolefin resin or film having high heat sealing capacity.
In order to improve the above mentioned drawbacks, in some examples 2,6-naphthalenedicarboxylic acid has been used instead of terephthalic acid, however, there has not been any example at all in which a polyester using an aliphatic dicarboxylic acid as the dicarboxylic acid has been molded into sheet, film or fiber form for practical use.
One of the reasons why the above mentioned polyester has not been practically used is that even if the aliphatic polyesters have crystallinity, most melting points of the above-mentioned aliphatic polyesters are 100.degree. C. or lower and they also have poor heat stability when they are melted. Of further importance is that the properties, particularly mechanical properties such as tensile strength, of these aliphatic polyesters are of an extremely low level; the polyester having the same level of number-average molecular weight as that of the above-mentioned polyethylene terephthalate shows much poorer properties, thus practical uses have not been found at all for these aliphatic polyesters.
It seems that the studies to possibly improve the physical properties of the aliphatic polyesters by increasing their number-average molecular weights have not made enough progress because of their poor heat stability.
Polyesters such as polyethylene terephthalate having no biodegradability, had a problem of requiring combustion treatment for their complete disposal, as simply disposing of it after use allows it to remain for a long time without being decomposed.
Particularly, in the packaging field, development of a film having high transparency, heat sealability, biodegradability for easy disposal and low combustion heat as well as high strength has eagerly demanded.
The object of the present invention is to provide polyester film, such as T-die flat film, air-cooling inflation film, water-cooling inflation film prepared by using aliphatic polyesters as their components which have sufficient high molecular weights for practical use, and excellent mechanical properties represented by thermal stability and tensile strength, and which can be decomposed by microorganisms and the light, and thus can be easily disposed of which provides only small amounts of combustion heat in disposal treatment after use, and which naturally possess heat sealing properties.